The invention relates to cooking appliances and, in particular, to a pressure fryer designed to prevent accumulation of condensed moisture within the cooking fluid.
Pressure cookers are well known in the field of cooking appliances. In such pressure cookers, the pressure inside a cooking vessel is increased above atmospheric pressure to cook food quickly while also retaining moisture in the food. Items such as chicken and fish are commonly cooked in this type of appliance. The food is often first coated with a breading and then cooked in a cooking fluid, such as oil, for a predetermined time period. Near the end of the time period, or cooking cycle, a valve is opened releasing the pressure inside the fryer so that the food items can be safely removed therefrom.
During the cooking process, small particles of the breading typically used to coat the food items being cooked often become dislodged and fall to the bottom. If these particles, commonly referred to as xe2x80x9ccracklingsxe2x80x9d, remain in the cooking oil during subsequent cooking processes, they can burn and contaminate the cooking oil.
In order to prevent such contamination, many prior cooker designs have incorporated what is commonly termed as a xe2x80x9ccold zonexe2x80x9d. This cold zone typically comprises a deep trough formed in the bottom surface of the cooking pot to collect the cracklings which accumulate during operation. The burners or heating elements are typically positioned well above the cold zone such that the zone is maintained at a temperature substantially lower than the cooking temperature. In this manner, cracklings may accumulate during successive cooking operations without burning and contaminating the cooking oil.
An example of such a pressure cooker is shown in U.S. Pat. No. 4,930,408 to King. The pressure cooker comprises a cooking vat divided into an upper cooking zone and a lower cold zone interconnected by a tapered annular wall. The cold zone is substantially smaller than the cooking zone, preferably comprising 20-30% of the volume of the vat. The cooking oil is heated by heating elements which line the walls of the vat in the lower portion of the cooking zone. The cold zone is maintained in essentially quiescent condition and at a substantially lower temperature than the temperature of the oil in the cooking zone to minimize cooking of cracklings and other contaminants.
U.S. Pat. No. 4,273,991 to Barnhill discloses a pressure fryer including strip heaters on the front wall, rear wall, and side walls of the fryer. The bottom edge of each lower most strip heater on each of the walls is spaced above the bottom edges of the tank""s side walls to create a relatively cold zone for the cooking liquid within the tank""s trough. Food particles which fall to the tank floor migrate toward the center of the floor since it is sloped downwardly relative to the bottom edges of the side wall, and are less likely to turn to charcoal.
U.S. Pat. No. 1,990,351 to Shroyer discloses a grease kettle including heating element which extends substantially through 270xc2x0 of arc of the circumference of the kettle and at its ends is provided with terminal portions which extend through the bottom of the kettle. A second heating element is arranged into a series of vertical convolutions spaced at intervals about the portion of the periphery of the kettle and is provided with terminal portions extending through the bottom wall of the kettle. The patent discloses that by reason of the arrangement of the heating elements, grease in the lower portion of the kettle over substantially the entire bottom surface will remain at a temperature below that at which food sediment will char or burn. At the same time, grease at the frying level will be heated to the frying temperature.
In these types of pressure fryers, the cracklings which become dislodged from the food being cooked naturally contain moisture. During subsequent cooking cycles, this moisture collects and condenses in the cold zone generally without problem. However, if the cooking process is interrupted for a short period of time between cooking cycles, oil stratification may occur. To provide a uniform oil temperature prior to resuming cooking operation, the oil is typically stirred. In the known pressure fryers described above, when stirred entrapped water in the cracklings contacts the cooking oil which is at temperature greater than the boiling point of water, 212xc2x0 F., and can be as high as 350xc2x0. The water thus converts to steam, causing a boil over condition which can injure the operator of the cooking appliance. Boil over also results, due to the same effect, from the naturally occurring convection currents within the oil which cause entrapped water to move up into the hot oil.
Canadian Patent No. 622,425 to Kelton discloses a pressure cooker including a cylindrical portion at the bottom of the vessel which constitutes a sump or sediment chamber into which fall particles of food. An elongated metal equalizer mounted in the center of the vessel is intended to conduct heat from the hot fat downwardly into the sump zone thereby keeping the sump above the boiling point of water, but also maintaining it below the burning point of the crumbs. However, in practice, this design has not proved successful. Also, the addition of such an equalizer to the interior of the cooking vessel limits the volume available for cooking and further restricts the use of baskets or racks typically placed in the vessel to support food items.
Another problem associated with the use of a cold zone is that as the water evaporates from the cracklings there is a period of time during which it passes through the cooking oil. When the cook cycle is complete and the pressure relief valve is opened, these small bubbles of water evaporate. Such evaporation results in a sudden undesirable temperature drop in the cooking oil.
The present invention provides a unique pressure fryer design in which the shape of the fryer and the placement of the heaters which heat the cooking liquid therein act to provide a cool zone at the bottom of the fryer. Within this cool zone, the temperature is maintained above the boiling point of water such that moisture contained within food particles is evaporated and the above-described boil-over condition is prevented. In addition, the temperature within the cool zone is kept well below the cooking temperature such that the food particles do not burn and contaminate the cooking fluid.
In accordance with the present invention, the pressure fryer includes a vat for containing cooking fluid. Preferably, the vat has a gently sloping bottom surface surrounded by a plurality of side walls. The bottom surface includes a central flat portion defining a lowermost point on the bottom surface. A plurality of heating elements are positioned on the side walls of the vat a sufficient vertical distance above the lowermost point on the bottom surface to form a cool zone between the heating elements and the bottom surface. In this manner, the temperature within the cool zone is maintained below the cooking temperature, but above the boiling point of water.
Another object of the invention is to prevent and/or control a sudden unwanted temperature drop in a pressure cooker caused by the sudden release of pressure and evaporation of water in the cooking oil. It is also noted that the invention has applicability in any environment where it is desirable to control the sudden decrease in temperature due to a pressure decrease.
In accordance with this aspect of the invention, a method and apparatus are provided for releasing the pressure from within the fryer and eliminating or minimizing the associated temperature drop. In accordance with the invention, a pressure release valve is operatively connected to a computer controller, which includes, as one of its inputs, output signals provided by a temperature sensor or sensors which are used to detect the temperature and rate of change of the temperature of the cooking oil. Near the conclusion of the cooking cycle, the pressure release valve is opened by the controller and the pressure within the fryer is released. The temperature and rate of change thereof are monitored by the controller. When the temperature of the oil detected drops to a predetermined temperature level and the rate of change detected by the controller indicates that the temperature is decreasing, the pressure release valve is closed and the cooking medium is allowed to absorb more heat. When the temperature rate of change stabilizes and/or the temperature of the cooking oil increases slightly, the pressure release valve is once again opened. The process is repeated at frequent intervals, with each opening of the pressure release valve allowing a small amount of pressure release and shutting to stop the release of pressure from the fryer when the temperature decreases and falls below a predetermined level.
Thus, the resulting arrangement provides for pressure release as quickly as possible while maintaining the temperature of the cooking medium at or near a predetermined set temperature. The intermittent release of pressure prevents undesirable rapid temperature drops.